The isolation and characterization of human embryonic stem (ES) cells permits an exciting new approach to reach the goals of regenerative medicine. ES cells can be maintained virtually indefinitely as undifferentiated cells in culture, yet retain the ability to differentiate to form any cell type in the body. Therefore, human ES cells will provide an excellent source of cells to replace damaged or degenerated cells and tissues to treat a host of diseases. In the area of hematology, derivation of hematopoietic stem cells (HSCs) from human ES cells may be used for hematopoietic cell transplantation therapies for patients with hematologic malignancies such as leukemia or myeloma. Production of mature blood cells may provide an unlimited supply of red blood cells, platelets and other blood products from this well-defined source that is free of viral or other contaminants. Already, in vitro studies with human ES cells have produced hematopoietic, neural, cardiomyocyte, and other cell types. Hematopoietic cells of myeloid (white blood cells), erythroid (red blood cells), and megakaryocyte (platelet precursors) lineages can all be produced by co-culture of human ES cells with irradiated bone marrow stromal cells. To reach their full potential, it is crucial to now demonstrate in vivo function of human ES cell-derived cells. This proposal aims to define the in vivo potential of hematopoietic cells derived from human ES cells by transplantation of undifferentiated human ES cells, and human ES cells induced to form hematopoietic precursors in vitro, into immunodeficient (NOD/SCID) mice. Use of this xenotransplant model to demonstrate human SCID-repopulating cells (SRCs) is an effective measure of hematopoietic stem cells with long-term regenerative potential. Genetically modified (GFP-labeled) hematopoietic cells derived from human ES cells will also be used to demonstrate the stability of foreign gene expression, and the potential use of this model as a vector for gene replacement therapies. Serial transplantation experiments into secondary and tertiary hosts will be done to demonstrate the ability of HSCs derived from human ES cells to sustain long-term self-renewal. Since rhesus monkey ES cell are also available, successful completion of the aims of this proposal will lead to a non-human primate model of ES cell-derived hematopoietic cell transplantation. Eventually, clinical trials to define human ES cells as a novel source of cells for hematopoietic cell transplantation therapies will open new avenues to treat hematopoletic malignancies, immunodeficiencies, autoimmunity, and many other diseases.